Kamala Harris and Donald Trump neck and neck in Wisconsin and Michigan: Polls

Donald Trump on Saturday called Democrat Kamala Harris "mentally impaired" and said illegal immigrants are out to cut the throats of ordinary Americans in their own homes as he doubled down on the racially charged rhetoric fueling his unprecedented bid to regain the US presidency.

Trump was seeking to strike back at Harris after she visited the US-Mexico border on Friday and vowed to do more to control asylum claims and undocumented migrant crossings.

The issue is one of the Democratic vice president's weakest in polling, as she runs neck-and-neck against Trump in the November 5 election.